The present invention relates to an electrophotographic printer, and more particularly, to a type thereof which can effect printing on leading and trailing portions of continuous web-like printing sheet without unprinted or idle zones at these portions.
One conventional electrophotographic printer is shown in FIG. 1. A tractor 21 is drivingly connected to drive motor 22 for feeding a printing sheet (not shown) toward a printing section. The printing section includes a photosensitive drum 1, a charter 2, a developer 4 in which toners ere accumulated, and an image transferring means 7 such as transferring corotoron. These charger 2, developer 4 and the transferring means 7 are disposed around the photosensitive drum 1. An electrostatic latent image is formed on the photosensitive drum 1 by a laser beam, light emitting diode array, or combination of a light source and a liquid orvstal shutter etc., and a toner image corresponding to the latent image is formed on the drum 1. The toner image is then transferred onto the printing sheet by the transferring corotoron 7.
At the downstream side of the printing section, a fixing unit is provided which includes a flash lamp 20. By the fixing unit, toner image outputted on the printing sheet is thermally fixed. Further, a drive motor 17 is provided for driving a transfer roller 19. The transfer roller 19 has a constant peripheral speed slightly faster than the sheet delivering speed given by the tractor 21 so as to avoid sheet slack. At an exit side of the printer, a sheet stacker 18 is provided into which the continuous printing sheet is successively folded and stacked.
A sheet guide member 6 is disposed at an inlet portion of the photosensitive drum 1. The sheet guide member 6 is positioned at both upstream and downstream sides of the transferring corotoron 7, and is pivotally supported to a frame body (not shown) by a pivot shaft 8. Therefore, the guide member 6 is movable toward and away from the photosensitive drum surface about the pivot shaft 8 so as to provide intimate contact of the printing sheet with the drum 1. Further, the sheet guide member 6 has an intermediate portion pivotally connected to one end of a link ii. The link 11 has another end pivotally connected to a cam 10 which is driven by a motor 9. Therefore, the sheet guide member 6 is movable toward and away from the outer peripheral surface of the photosensitive drum 1. A gap having a distance of about 0.5 mm is maintained between the photosensitive drum 1 and the sheet guide member 6 so as to maintain intimate contact between the printing sheet end the drum 1. Such small gap can withstand tension applied to the sheet during printing. Incidentally, desirable image transfer cannot be achieved unless the sheet is in intimate contact with the photosensitive drum. In this connection, the gap distance between the sheet guide member 6 and the photosensitive drum 1 must be within a range of 0.5 mm to 1.2 mm. In case of the cut sheet, the sheet can be sticked onto the photosensitive drum because of the electrostatic force. Therefore, the gap distance of not less than 0.8 mm is also available. However, in case of the continuous sheet, the gap distance must be not more than 0.8 mm, otherwise the sheet may be displaced from the exact printing region, since precedent portion of the continuous sheet is held by the transfer roller 19 and therefore the tension is imparted on the sheet.